1. Field of The Invention
The invention relates to a tuner having an input for receiving an input signal, said tuner comprising an oscillator for generating an oscillator signal, dividing means for dividing the oscillator signal, and a mixer for mixing the input signal with the oscillator signal, and an output for supplying an output signal. The invention relates more particularly, but not exclusively, to a global FM tuner.
The term tuner should herein be interpreted as a tuner-integrated circuit and as a tuner-module comprising one or more integrated circuits and other discrete components.
The invention further relates to a method of a receiving an input signal, comprising the steps of generating an oscillator signal, dividing the oscillator signal, mixing the oscillator signal with the input signal and supplying an output signal.
2. Description of The Related Art
There are, for example, different standards for the FM reception bands across the world. The European and U.S. standards differ in their channel spacing and pre-emphasis, but have approximately the same frequency limits, 87.5-108 MHz in Europe and 87.9-107.9 MHz in the U.S.A. The channel spacing and band limits may be software-programmable parameters. In some stereo decoders, the de-emphasis value is, also a software-controlled parameter, for example, the CASP IC TEA6880 of Philips Semiconductors.
Therefore, no hardware change is necessary for a U.S.A./Europe FM tuner, i.e., an FM tuner which can be used in Europe and in the U.S.A.
However, this is not the case with the Japanese and Eastern European frequency bands. In Japan, the FM band ranges between 76 and 91 MHz. The oscillator frequency is set below the frequency of the requested channel to avoid TV transmission frequencies at the image position, i.e., IF=xe2x88x9210.7 MHz. Therefore, a global tuner would require a very large range to cover the Japanese band as well as the others; a VCO frequency range from 65.3 MHz to 118.7 MHz. As those skilled in the art know, this requires a capacitance change in the varactor diode of more than 3.3. In practice, due to stray capacitances, a capacitance ratio of more than 4 would be required.
Unfortunately, however, the tuning voltage is often limited. For example, in car radio applications, the supply voltage is only 8.5V, which can drop to 8V due to ripple and/or stabilizer tolerances in the supply lines. The saturation voltage of the known PLL charge pump further reduces the available tuning voltage to about 7.5V. However, to maintain good tracking and linearity of the tuner front-end of the receiver, the lower limit to the tuning voltage should not be far below 2V. The remaining tuning voltage range is too narrow to achieve the required capacitance change with varactor diodes in their application at these frequencies. Therefore, the standard approach is to use different components in the tuner front-end for the FM-Japan application.
The frequency of the undesired signal is referred to as image frequency or image signal.
The above problem can be solved, as has been done in the IC TEA6840 of Philips Semiconductors, by setting the oscillator to above the requested channel, also in the Japan band, which reduces the frequency range to 87.6-118.7 MHz, and by employing integrated image injection to reduce interferences by the TV transmission frequencies at the image frequencies.
The oscillator is operated at twice the required frequency to obtain, after a division by 2, a phase shift which is required to obtain an oscillator signal with a phase difference of 0 and 90 degrees.
The TEA6840 does provide a global tuner application, but still suffers from a number of drawbacks. Even with 70 dB of image rejection, FM reception in some areas in Japan is inferior when compared to receivers with the oscillator frequency below the requested channel, where there are no TV transmission frequencies at the image frequencies (as is standardized in Japan). Furthermore, the tuning voltage for the lower limit of the Japanese
band is only about 1 V. This low tuning voltage reduces the third-order intermodulation performance of the tuner front-end.
This problem is even greater in the Eastern European band where the lowest tuning voltage is only 0.3V. In this case, the oscillator frequency is not required to be below the requested channel, but the reception band starts at the very low frequency of 65 MHz.
It is an object of the invention to provide a global tuner which does not have the drawbacks of the tuners described above and would further tune to the Japanese band with the oscillator below the requested channel without any hardware change and with an acceptable minimum tuning voltage. To this end, a first aspect of the invention provides a tuner as set forth in the opening paragraph, characterized in that the dividing means is switchable between at least 2 values. A second aspect of the invention provides an audio receiver comprising such a tuner. A third aspect of the invention provides a method of receiving a signal.
By using dividing means which can be switched between at least two values, it is possible to use a division by, for example, a factor of 2 for Europe and the U.S.A., and to use, for example, a factor of 3 for Japan and, for example, Eastern Europe. This makes it possible to use IF=xe2x88x9210.7 MHz for Japan because the division factor of 3 brings the actual oscillator frequencies, and hence the tuning voltages, to roughly the same range as the European and U.S. bands.